Volume processing of products with microwaves, such as needed for large scale cooking and the like, typically uses a transmission-type system in which the microwave energy and the product being processed travel in the same direction through the same waveguide. This was an early approach by Marshall, L. K., “High Frequency electromagnetic cooking apparatus”, U.S. Pat. No. 2,495,415 (1950) and in Revercomb, H. E. and Watts, D. E., “Ultrahigh frequency dielectric heater”, U.S. Pat. No. 2,467,230 (1949). Later refinements by Johnson, R. M., “Waveguide applicator and method”, U.S. Pat. No. 3,597,565 (1971) and Bleakley, W. J., “Waveguide”, U.S. Pat. No. 3,555,232 (1971), involved making changes to the shape of the waveguide to improve uniformity of product heating.
However, all of the processes involving coincident microwave energy and product flow have problems when the product to be processed has a high loss factor of microwave energy, and is of sufficient cross section to provide substantial attenuation of the energy in a short distance. This combination results in instantaneous heating where the product and microwave first co-mingle, and also results in substantial non uniform heating when the product is relatively wide and thick. Having the product flow opposite to the microwave flow does not substantially improve uniformity where the energy attenuation is such that most energy absorption occurs in a short distance. This distance can be as small as 6–12 inches in WR975 type waveguide at 915 MHz, for example.
Another commonly used waveguide applicator was first described by Stiefel, K. J., “Waveguide dielectric heating apparatus”, U.S. Pat. No. 2,560,903 (1951). This moves the product through slots in the major waveguide walls in the direction of the microwave electric field, such that the product flow and microwave flow are at right angles. This device is most applicable to heating a thin sheet-like material and can be typically provides relatively low attenuation. Multiple passes through the waveguide, which can be configured in some form of meander line to expose the material to the microwave on multiple occasions, is required for efficient operation using this method.
Many refinements have also been described for processing of web-like materials lying in the electric field plane, such as Smith, F. J. and Silberman, K., “Method and Apparatus for drying sheet materials”, U.S. Pat. No. 3,622,733 (1971); Gaon, D. and Wiedersatz, J., “Process for preparing fat free snack chips”, U.S. Pat. No. 5,180,601 (1993); and Hedrick, et al. in “System for applying microwave energy in processing sheet like materials”, European Patent Application No. 0667732A1 (1995). These techniques are all applicable to thin or web-like materials with relative low absorption.